Adding a new feature in Docs or Sheets is easy enough: Select "Add-ons" from the main menu, and choose an add-on from the gallery. Add-ons are installed server-side, not client-side, so they're not tied to any particular client. Right now, the gallery is small, with about 30 products currently available, but Google's blog post on the subject hints at many more on the way.

Most of the debates about Google Docs/Sheets revolve around whether the app can ever completely replace the likes of a local app like Microsoft Office or OpenOffice.org/LibreOffice. The add-on system likely won't change the picture much, but at least it helps plug many of the functionality holes that have shown up in Google Docs and Sheets over time.

One possible downside to add-ons: It might give Google that much less incentive to improve core Docs and Sheets products. If Google comes to believe whole swaths of functionality are best left to third parties, end-users may find themselves beholden to not just one but multiple third parties for the sake of accomplishing everything they need.

It's also hard not to worry about what happens if a given add-on is ditched by its own company. If it were a physical software product, it could still continue to be used in some form, even if it lacked for support or new features. Odds are any Google Docs/Sheets add-on that's discontinued by its creators will simply cease to exist.

Still, some fairly major names are providing many of the add-ons -- reassuring news. The label merge app is courtesy of Avery, for example, and the mail-merge add-on is by Mailchimp. And the overall convenience of Google's app system, with nothing to download and nothing to install locally, ought to appeal to users and companies with modest needs.